Prior Art—U.S. Patents
Prior Patents Indicating Recycling from Synthesis Unit to a Partial Oxidation Gasifier, Reformer, Autothermal Reformer, or Equivalent.
Chang, et al. (U.S. Pat. No. 4,138,442; 1979) starts with a syngas from fossil fuels that is converted by catalyst to a mix of methanol and DME. The DME is converted with a zeolite catalyst to product containing gasoline and a light hydrocarbon gas fraction. The latter could be liquefied, releasing a hydrogen gas that could be recycled to the fossil fuel conversion or otherwise used.
Iijima (U.S. Pat. No. 6,489,370; 2002) uses steam mixed with natural gas to generate a syngas in a reformer heated by radiation from a separate combustion unit. Carbon dioxide from the combustion unit is added to the steam-natural gas mixture into the reformer to generate more carbon monoxide. The unit acts as a partial oxidation reformer, but with indirect heating so that air can be used as the oxidant.
Thiebaut (U.S. Pat. No. 6,846,951; 2005) recycles 5-50% of the carbon dioxide from a reformer and fed with a natural gas feedstock and oxygen into an autothermal reformer in a process to make methanol and acetic acid.
Fenouil, et al. (U.S. Pat. No. 7,250,450; 2007) uses a gaseous hydrocarbon feedstock in a partial oxidation process to make a syngas fed to a catalytic converter. The resulting hydrocarbon products are separated from a recycle stream. Carbon dioxide is then separated out from the recycle stream before recycling the carbon dioxide depleted stream into the partial oxidation unit.
Gueh (U.S. Pat. No. 8,513,316; 2013) presents ideas of mixing carbonaceous feedstock with recycled carbon dioxide and tail gas from FT synthesis to feed a thermal conversion plant or a chemical conversion plant to generate a syngas. The thermal conversion plant consists of a molten metal bath heated externally. The ideas are not supported by any data or details on how to accomplish the proposed methods.
Ravikumar, et al. (U.S. Pat. No. 8,629,188; 2014) proposes a gas to liquids plant in which energy for natural gas reformation is provided at least in part by biomass (shale oil) gasification. A hydrocarbon feed stream is provided to a reformer and a second biomass feed stream partially oxidized. The latter is partly fed to a burner for the reformer energy and then the balance combined with the first feed stream to be synthesized in a Fischer-Tropsch process or equivalent into a liquid fuel product. Carbon dioxide in the product stream is redirected to the reformer. The water effluent is separated into a waste water that is combined with the first feed stream and into a purge stream, a portion of which is fed to a turbine to generate power, the exhaust energy of which is recycled to the reformer.
Blevins, et al. (U.S. Pat. No. 8,936,769; 2015) produce a syngas from a mix of steam and carbonaceous material in a reformer. A catalytic conversion unit then generates a hydrocarbon product stream from which a tail gas is separated of carbon monoxide, carbon dioxide, hydrogen and methane. Tail gas options include possible fluid flows to various system components such as a mixing apparatus, a reformer, and/or catalytic synthesis conversion unit.
Prior Patents Indicating Recycling from a Synthesis Unit Back to a Synthesis Unit (a Common Way to Increase the Carbon Conversion Efficiency for a Given Catalyst) or Equivalent.
Janda (U.S. Pat. No. 6,444,712; 2002) proposes a methanol synthesis unit and a hydrocarbon synthesis unit to generate methanol and hydrocarbon products from natural gas. Carbon dioxide from the hydrocarbon synthesis unit is separated out and mixed with natural gas to obtain a optimal syngas composition to the separate methanol synthesis unit.
Price (U.S. Pat. No. 6,740,683; 2004) synthesizes chemicals from syngas. Uses hydrogen-poor hydrocarbons with H2/CO<2, but recycles vapor product from the FT synthesis process to obtain 2<H2/CO<3 (a typical FT synthesis recycle technique).
Early (U.S. Pat. No. 7,790,775; 2010) presents complex methods of recycling syngas and FT synthesis products streams to the synthesis process to increase carbon conversion efficiency.
Severinsky (Patents U.S. Pat. No. 7,641,292; 2010, U.S. Pat. No. 8,114,916; 2012 and U.S. Pat. No. 8,168,143; 2012) presents methods of recycling syngas and FT synthesis products streams to the synthesis processes to increase carbon conversion efficiency.
Menzel (U.S. Pat. No. 8,741,971 B2; 2014) discusses a method and system for operating a Fischer-Tropsch synthesis from coal gasification for production of feed gas of CO and hydrogen. The gas is desulphurized and fed to the Fischer-Tropsch system for the production of liquid products. The CO and CO2 gas exiting the FT process is compressed and fed to a convertoer stage in which the CO is converted with steam into H2 and CO2. The CO2 is subsequently removed and the H2 enriched gas is recycled along with primary de-sulphurized gas back to the FT process. Advantage is seen in reduced de-sulphurizing costs and increased H2 content of the gas entering the FT plant.
Prior Patents Indicating a Separation but No Recycling for Synthesis, or Equivalent.
Bohn, et al. (U.S. Pat. No. 6,306,917; 2001) use a partial oxidation process to generate a syngas from a hydrocarbon feedstock. After FT synthesis, the CO2 is separated out as a product (not recycled to system) and the remaining hydrogen rich tail gas used in a gas turbine to generate power.
Prior Patents Using External Sources of Carbon Dioxide
Shiroto, et al. (U.S. Pat. No. 6,656,978; 2003 and U.S. Pat. No. 6,806,296; 2004) uses lower hydrocarbon numbers and an external source of carbon dioxide mixed with steam. A special catalyst produces a syngas with a carbon conversion efficiency of at least 50%. If the syngas molar H2/CO ratio is 1.5 to 2.5, syngas is reacted in a FT catalyst process to generate liquid oil. If 0.5<H2/CO<1.5, react with catalyst to synthesize methanol or DME.
O'Rear (U.S. Pat. No. 6,774,148; 2004) blends final syngas from two syngas sources. First source is from a partial oxidation reaction of methane and oxygen to get H2/CO>2. Second source is from LPG and external carbon dioxide to synthesize a syngas of H2/CO>1.5.
Wolf (U.S. Pat. No. 7,960,441; 2011) combines carbon dioxide from combustion and hydrogen from electrolysis in a high temperature system to generate a syngas prior to conversion to hydrocarbon fuels.
Shulenberger et al. (U.S. Pat. No. 8,198,338 B2; 2012) discusses the production of high octane fuel from carbon dioxide and water. Feedstock consists of industrial carbon dioxide and water with the consumption of electricity to drive the process. End products include high octane gasoline, high cetane diesel, or other liquid hydrocarbon mixtures. The process primarily depends on the electrolysis of water into hydrogen and oxygen for initiation. Secondary processes include mixing of hydrogen with CO2 to optimize the conversion of CO2 to CO, mixing of hydrogen with CO to produce syngas for conversion to methanol or other hydrocarbons, synthesis of methanol, conversion of methanol to dimethyl ether (DME), conversion of DME to gasoline, conversion of DME to diesel, synthesis of gasoline directly from methanol, and synthesis of DME from syngas.
Surma et al. (U.S. Pat. No. 8,685,121 B2; 2014) discusses the processing of heterogeneous feedstocks including organic and inorganic material in a gasification/vitrification unit. The process includes a downdraft gasifier coupled to a vitrification unit which is then further coupled to a thermal residence chamber. Feedstocks are introduced into the gasifier and mechanically transferred downward with the volatile and non-volatile fractions exiting the gasifier vertically downward through a grated port into a high temperature joule heated vitrification unit. The vitrification unit is controlled at a higher temperature than the gasifier for further processing of the non-volatile fraction into a molten glass product, and the volatile fraction further processes in the vitrification headspace via exposure to plasma from torches or electrodes. The volatile fraction exiting the vitrification unit to a downstream thermal residence chamber operated a third temperature. The gasification process utilizes oxidants in the form of pure oxygen (90-99% pure), air, carbon dioxide, oxygen enriched air, steam or a combination thereof to maintain a reducing environment for the production of high quality syngas.
Other References
METSIM pyrometallurgical software. Proware, Tucson, Ariz.
Zennaro, et al., Syngas: The Basis of Fischer-Tropsch chapter, p. 38, Greener Fischer-Tropsch Processes, P. M, Maitliss & A. de Klerc, Eds., Wiley-VCH, Weinheim, Germany, 2013.
DOE 2000 report: Natural Gas to Liquids Conversion Project, Raytheon Engineers & Contractors, DOE Report 2000-1032585, 2000.
M. McKellar, et al., Aspen Process Model for the Misty Mountain Resource Recovery Plant, INL TAP Report INL/LTD-15-36850, October 2015.
Miglio, Zennaro and de Klerk, Environmental Sustainability chapter, p 329, Greener Fischer-Tropsch Processes, P. M, Maitliss & A. de Klerc, Eds., Wiley-VCH, Weinheim, Germany, 2013.
Discussion of Issues
The processing of heterogeneous waste materials, such as municipal solid waste (MSW) or construction and demolition waste (C&D), into a syngas of H2 and CO2 for conversion to liquid fuels is difficult by combustion methods. The variability in flammability and heats of combustion, as well as inorganic content and inertness makes these wastes difficult to burn, as well as trying to partially oxidize properly to a syngas, without sorting organic materials out from metals and inorganic (soil, glass, bricks, ash, etc.) materials.
The utilization of the Fischer-Tropsch catalytic synthesis (FTS) of hydrocarbon liquid fuels (HCLF) has focused on converting natural gas/methane or coal. Various forms of partial-oxidation are employed to obtain the energy for high temperatures required for gasification to a syngas of CO and H2 and to limit formation of CO2 and H2O. Pure oxygen and steam are usually added to make up defficiencies of oxygen and hydrogen in feedstock. Traditional preference is to obtain a molar H2/CO ratio of 2 or larger so that in the FTS, oxygen released from CO joins with excess H2 to make a waste effluent of water, rather than CO2. If insufficient H2 is introduced, CO2 is formed as a waste effluent, thus reducing the HCLF yield by reducing carbon conversion efficency.
A result is that focus of partial-oxidation generation of syngas has been on adding pure oxygen and/or steam to various feedstocks of CH4, coal, biomass or MSW. This is a major issue for heterogenious wastes because of the added amount of additives that must be added to obtain H2/CO>2, and the energy required to disassociate these additives toward gasification.
Another issue is that biomass (wood chips, etc.), MSW and C&D wastes have lower heats of combustion making it difficult to reach gasification temperatures with partial-oxidation requirements on CO and H2. Lower temperatures produce more complex hydrocarbon species that complicate the downstream cleanup process for the FTS.
A third issue is that biomass and MSW contain various amounts of moisture which lower the heat of combustion and temperatures even further, if not dried. Drying requires additional energy. If dried, steam needs to be added later to provide for the deficiency of hydrogen in waste requiring more energy input.
A fourth issue is that whereas biomass may be relatively homogenious, MSW and C&D wastes are not, containing organic material, inorganic material and metal. In a partial-oxidation process the organic materials need to be pre-sorted out to make the syngas.
Fifth, the heterogeneous nature of the MSW and C&D wastes makes it difficult to control the partial-oxidation process temperatures and reactions in the reaction chamber to obtain a consistant syngas composition with a minimum of undesireable hydrocarbon species.
A sixth issue is that there is always some ash generated. Temperatures from partial-oxidation methods are insufficient to obtain a molten, homogenious ash or slag that can be used to produce value-added construction products, rather than aggregate. The same temperature constraints can be said for any metal waste in the feedstock, reinforcing the need for sorting.
Syngas generation methods considered have usually been a form of partial-oxidation methods, the variations of which will be considered as one here. An alternative gasification method is by using electric furnace methods, using a variety of plasma torch melter (PTM) or graphite-electrode arc melter (GAM) furnaces. The focus of this work is on a form of the electric arc furnace (EAF) or GAM called a submerged arc furnace (SAF) with which the electrodes are immersed into the molten slag in the furnace for optimum performance.
An issue with the electric furnace process is cost of electricity compared to cost of partial combustion with pure oxygen. This would be a major factor if one follows the conventional practice of adding steam and other additives to raise the molar H2/CO>2 in the syngas for those wastes having H2/CO ratio near unity. Economic studies have shown that processing of MSW in a SAF compensates for the electricity costs by way of revenues from value-added products. The SAF processes are competitive if not more economic than other processes for waste-to-energy plant sizes larger than 500 tonnes/day.
Of the prior art U.S. patents, only the first section on recycling additives into a gasifier, reformer or similar unit prior to a synthesis conversion unit appears to be comparable to the embodiments proposed here. Most of those employ partial oxidation methods which are not used herein because of the heterogeneous nature of the feedstock. When CO2 is added to the process it is usually because the main feedstock is natural gas with an abundance of hydrogen so that the water gas shift can be used at lower temperatures to shift CO2+H2 to CO and H2O to give more carbon monoxide for conversion in the synthesis process. Many of the processes adding CO2 were generating alcohols that contain the OH radical and can utilize the additional oxygen from CO2.
The most similar patent to this is that of Gueh (U.S. Pat. No. 8,513,316; 2013) that mixes CO2 and tail gases from a FT synthesis with a carbaonceous feedstock and passes the mixture through a molten metal bath as a medium to obtain a syngas without any discussion of compositions or thermal conditions. The metal must have a relatively low melting temperature to be melted by an external source in the marine vehicle, too low a temperature to melt the ash contained with the carbonaceous feedstock, unless it is a gaseous feedstock. It appears impossible if not impractical to do what the author proposes from the patent description which is not documented by ant data. The present embodiment uses a molten slag/ash to assist in the gasification process and processes the additives within the system to get maximum benefit from their capabilities.
Advantages
A Prior work (U.S. Pat. No. 6,204,427, CA 2274540) discusses a process and apparatus (P&A) that can separate the metal and inorganic materials from the organic materials, and from organic materials produce a syngas to generate liquid fuels in a FTS process. This prior work is the basic P&A on which the proposed embodiment is an extension. The proposed embodiment couples prior P&A to a FTS process in such a way as to compliment each other. Compared to partial-oxidation methods for MSW, C&D and biomass wastes, advantages of the coupled SAF-FTS are:                To generate a cleaner syngas with less complex hydrocarbon species.        To reduce external input-energy requirements.        To increase energy efficiency of the coupled systems.        To increase the HCLF yield above that from the input feedstock alone.        To increase the internal recycling of process effluents.        To decrease the carbon footprint.        To resolve the presented issues of the partial-oxidation methods for heterogenious waste.        